Continuity Nod: The USS Defiant is named for the ship of the same name in the TOS episode "The Tholian Web". That earlier Defiant would go on to reappear in the Enterprise episode "In a Mirror, Darkly".
Curbstomp Battle: Once again, the Jem'Hadar have Starfleets number when it comes to combat.
Enemy Mine: The Federation takes the Dominion threat seriously enough to do a deal with the Romulans, acquiring a cloaking device for the Defiant at the price of having a Romulan observer on board.
No One Gets Left Behind: A rare aversion when Sisko is forced to abandon Dax and O'Brien when they're captured by the Jem'Hadar.
Skewed Priorities: Quark suggests Rom accompany Sisko on the mission instead. After all, he just has a son. Quark has a business.
Taking A Third Option: This trope is directly quoted by Kira after the senior staff (minus Sisko) has run many simulations and found that the station will be overrun by the Dominion very quickly in the event of a full-scale assault. Dax says that that leaves them with two options— abandon the station and make their stand on Bajor, or collapse the entrance to the wormhole. Kira says, "I want a third option" and almost at that very moment, the third option appears in the form of the new USS Defiant.